DESCRIPTION: The molecular basis of mucopolysaccharidosis I (MPS I, the Hurler, Hurler-Scheie and Scheie syndromes) is a mutation in the gene encoding alpha-L-iduronidase, resulting in deficiency of the enzyme and lysosomal accumulation of undegraded substrate. Because a lysosomal enzyme can be secreted and also taken up by endocytosis, MPS I is a good candidate for therapy by exogenous administration of the gene or enzyme. Previous trials of recombinant alpha-L-iduronidase administered to the canine model of MPS I resulted in promising biochemical, pathological and even clinical improvement, but were limited by lack of sufficient enzyme. Aim 1 is to scale up the present method of enzyme production from secretions of a stably transfected CHO cell line, primarily by growing the cells in larger volumes and to higher density under well controlled conditions. Aim 2 is to administer the recombinant enzyme in high dose to three affected MPS I dogs for a period of one year, in order to determine if the pathological and clinical manifestations of the disease can be significantly altered in the treated animals relative to untreated littermate controls, as well as to assess immunologic complications of the therapy. Aim 3 is to generate an alpha-L-iduronidase deficient mouse by homologous recombination technology and to characterize its phenotype, in order to make available a small animal model of MPS I. Aim 4 is to use the MPS I mouse for experiments that could not be performed in the dog: to improve the efficacy of enzyme replacement, to study enzyme uptake by isolated brain cells and to test the effectiveness of gene transfer to hematopoietic stem cells via retroviral vectors. Aim 5 is to construct a retroviral vector with a chimeric envelope protein for ligand-based targeting to cells bearing the GM-CSF receptor, in order to deliver alpha-L-iduronidase to myeloid precursor cells. The studies proposed in this application represent steps in an overall program to develop effective treatment for patients with MPS I.